What is a .prt file?

Two formats, one extension

The .prt extension is the least helpful in CAD, because the two largest high-end mechanical CAD systems both chose it, decades ago, for unrelated file formats. A Creo part and an NX part share three letters and nothing else. Before doing anything with a .prt, find out which it is.

A Creo file, and every Pro/ENGINEER file before it, carries a version number after the extension. The file on disk is bracket.prt.1, and each save writes bracket.prt.2, bracket.prt.3 and so on, keeping the old ones until someone purges them. Windows tends to show the last part as the extension, which confuses people, but the name is a reliable fingerprint. Inside, the file begins with a short text header that starts #UGC, and the rest is binary.

An NX file has no version suffix: bracket.prt is overwritten in place. It is binary throughout. NX assemblies are also .prt, holding references to the component parts, so an NX assembly is not visibly different from a part by name.

Unigraphics, the system NX grew out of, used the same extension, and older files from it still turn up. And CADKEY used .prt for its own format in the 1990s, which is what an unusually small .prt that nothing recognises often turns out to be.

What either one contains

Both are full parametric part files. The solid is the finished geometry from the kernel (Granite in Creo, Parasolid in NX), and behind it sits the feature history that built it, with sketches, dimensions and relations. Both store units per part, materials, colours and layers. Neither is documented for third parties, and neither system reads the other's files natively; each imports the other through a paid translator or through a neutral format.

Siblings

Creo assemblies are .asm, again with version numbers, and drawings are .drw. An assembly references its parts by name and searches configured folders for them, so an assembly sent alone will not resolve. NX assemblies, as above, are .prt themselves, and NX drawings are stored inside part files too.

Where these files come from

Aerospace, automotive and heavy equipment, where Creo and NX are the systems of record at the large manufacturers and their supply chains. An engineer at a smaller supplier receives these from a customer's PLM system, and the customer will nearly always provide a STEP on request, because they have the same translation problem in reverse.

Getting geometry out

From Creo: Save a Copy, and pick STEP, IGES, Parasolid, STL or others. From NX: Export, with the same choices plus JT, Siemens' lightweight visualisation format. STEP AP214 or AP242 from either carries the solid with colours.

Without the native system, eDrawings opens both kinds free on Windows and Autodesk Viewer displays a Creo part in a browser. Onshape's free plan imports both and exports STEP from what it imported, which is the no-cost route to a file this site can open. SolidWorks, Inventor and Fusion import both on paid licences.

How to open a .prt file

First find out which .prt you have, because two unrelated CAD systems use the extension. A Creo file (formerly Pro/ENGINEER) usually carries a version number after the extension, as in bracket.prt.4, and begins with the text #UGC. A Siemens NX file has no such suffix and is binary throughout.

eDrawings opens both kinds free on Windows, and Autodesk Viewer displays a Creo part in a browser. Onshape's free plan imports both. SolidWorks, Inventor and Fusion import both as geometry, Fusion on a commercial subscription only. This site cannot read either.

To edit with the feature history you need the original system: Creo Parametric for a Creo part, NX for an NX part. Neither reads the other's .prt. If a file refuses everything above, it may be a Unigraphics part from before NX, or a Solid Edge file that was renamed; ask the sender what wrote it and for a STEP export alongside.